In general, a water heater is an apparatus configured to heat cold water to a predetermined temperature within a short time so that a user can conveniently use hot water.
FIG. 1 is a schematic view showing a configuration of a conventional water heater.
Reviewing the configuration of the conventional water heater 10, a filter 1 configured to filter foreign substances included in tap water introduced therethrough to purify the tap water and a flow rate sensor 2 configured to measure a flow rate of the introduced tap water are installed on a tap water inlet pipe 5 through which the tap water is introduced, the tap water introduced into a heat exchanger 8 is heated by heat exchange with combustion heat generated by combustion of air supplied from a blower 6 and a gas by a burner 7 to be discharged through a hot water supply pipe 9, and a flow rate control valve 4 configured to control a flow rate of the hot water is installed on a hot water supply pipe 9.
In addition, a bypass pipe 5a configured to directly convey the introduced tap water to the hot water supply pipe 9, not through the heat exchanger 8, is connected between the tap water inlet pipe 5 and the hot water supply pipe 9 so that the hot water heated through the heat exchanger 8 is mixed with the tap water to control a temperature of the hot water.
Further, a mixing valve 3 is installed on the bypass pipe 5a to control a flow rate of the tap water conveyed through the bypass pipe 5a. 
FIG. 2 is a cross-sectional view showing the conventional pipe connecting structure of the water heater.
The conventional water heater 10 has a structure in which a flow path including the tap water inlet pipe 5, the hot water supply pipe 9 and the bypass pipe 5a is formed in the water heater housing 11, and a plurality of pipes are connected on the tap water inlet pipe 5, the hot water supply pipe 9 and the bypass pipe 5a to install the flow rate sensor 1, the flow rate control valve 4 and the mixing valve 3.
That is, the tap water inlet pipe 5 connected from the tap water inlet A to the heat exchanger 8 has a structure in which a plurality of connecting pipes 21a, 21, 22 and 23 are fastened to each other, the hot water supply pipe 9 connected from the heat exchanger 8 to a hot water outlet B has a structure in which a plurality of connecting pipes 24, 25, 26 and 26a are fastened to each other, and the bypass pipe 5a also has a structure in which a plurality of connecting pipes 22 and 25 are fastened to each other.
In addition, a fastening part between the connecting pipes is fixed by a fastening member 30 such as a clip, etc., to keep a fluid hermetically sealed.
As the conventional pipe connecting structure of the water heater is configured to connect the connecting pipes adjacent to both ends of the flow rate sensor 1, the mixing valve 3 and the flow rate control valve 4, as shown by {circle around (1)} to {circle around (12)} of FIG. 2, the number of fastening parts between the connecting pipes is excessive and a pipe structure is complicated, thereby increasing probability of leakage between the fastening parts.
Further, in order to perform maintenance of the flow rate sensor 1, the mixing valve 3 and the flow rate control valve 4, since the fastening parts {circle around (1)} and {circle around (2)} of both ends of the flow rate sensor 1, the fastening parts {circle around (3)} and {circle around (4)} of both ends of the mixing valve 3 and the fastening parts {circle around (9)} and {circle around (10)} of both ends of the flow rate control valve 4 should be separated, the maintenance cannot be easily performed and an operation thereof takes much time.